Ion mobility spectrometers are usually operated by injecting short ion current pulses into the drift tube. The ions are continuously generated in an ion source and then admitted into the drift region of the spectrometer by a gating grid over a short time span. The time spans for the admission usually amount to between 100 and 300 microseconds; the acquisition of the spectrum takes about 30 milliseconds. Bipolar grids are used as gating grids.
The ions admitted by the grid are drawn through a collision gas in a drift region by an axial electric field. The velocity of the ions depends on their “mobility”, which, as known, in turn depends on their collision cross-section, their mass, their polarizability and their tendency to form complex ions with molecules from the collision gas. In the ion source, several ion species, such as monomers, dimers and complexes with water and with the collision gas, are usually formed from the molecules of a substance. Every ion species has a characteristic mobility. At the end of the drift region, the incident ion current is measured at an ion detector, digitized and stored as a “mobility spectrum” in the form of a digitized sequence of measured ion current values. An evaluation of this mobility spectrum provides information on the mobilities of the ions involved and therefore indications for the substances involved.
The method is sensitive to certain groups of substances and is used on a large scale in the measurement of pollutants in air, for example, for monitoring chemical laboratories, continuous monitoring of filters, control of drying processes, monitoring of exhaust air, detecting warfare gases, and so on.
Ions with the same charge experience the same drawing force from the electric field, but as a result, ions with different collision cross-sections and different masses have different drift velocities through the collision gas. For lighter ions of about the mass of the collision gas, their mobility is determined mainly by the reduced mass of the ions, for heavier ions from several hundred or thousand atomic mass units upward, it is the particular form of the molecule that is decisive and the collision cross-section becomes significant.
The switching operation of the grid acts as the start time for measuring the drift velocity of the different bunches of ions. During the drift process, the diffusion of the ions in the forward and backward direction generates a diffusion profile for each bunch of ions containing ions of the same mobility. This results in a roughly Gaussian bell-shaped curve for the ion signals. The drift velocity is determined from the measured drift time in the center of the bell-shaped curve and the known length of the drift region in the drift tube of the spectrometer.
The ions of the analyte substances are regularly formed by so-called “atmospheric pressure chemical ionization” (APCI) in reactions with reactant ions by protonation or deprotonation, and dimeric ions and complexes involving water and collision gas molecules are formed in addition to monomeric pseudomolecular ions. “Pseudomolecular ions” are protonated or deprotonated analyte molecules and therefore have a mass that is increased or reduced by one atomic mass unit compared to a normal molecular ion. The ratios of the individual ion species with respect to each other depend on the concentration of the analyte molecules in the collision gas.
Nitrogen or air is usually used as the collision gas. The collision gas contains traces of water vapor, the concentrations of which are usually carefully controlled by filter units. The primary reactant ions are in most cases generated by irradiation from beta emitters, for example 63Ni, but corona discharges and other electron beam generators and UV lamps are also used for this purpose. The secondary reactant ions are formed in a reaction chain, which starts with the production of primary nitrogen ions and ends up with a number of different water complex ions. These water complex ions perform the actual chemical ionization of the analyte molecules by protonation or deprotonation.
As the analyte ions drift through the collision gas of the drift region, the ions quickly experience new attachments and losses of H2O water molecules and N2 nitrogen molecules. Statistically averaged, an analyte ion, whether it is a monomer or a dimer, thus contains a×H2O and b×N2, where a and b are usually non-integral numbers. The peak in the mobility spectrum is hardly broadened by this because these changes happen very quickly in a kind of dynamic equilibrium. If one investigates the ions of such a peak in a connected mass spectrometer, one freezes a momentary state, just like in a flash photograph, and obtains a mass spectrum which contains the ions with various states of attachment, and thus very different masses, side by side.
For a conventional repetition rate of the spectrum measurements of about 30 spectra per second, and an ion gating time of between 150 and 300 microseconds, the utilization factor of the ions of a substance introduced in a gaseous state amounts to only between a half and one percent. The remaining ions are discharged, predominantly in the gating grid, and are lost for any further measurement.
Ion mobility spectrometers are often compared to time-of-flight mass spectrometers because both are spectrometers with a time-of-flight dispersion. There are, however, major differences, which mainly concern the diffusion of the ions in the collision gas and thus the shape of the ion signals at the detector. Despite the large differences, some publications dealing with the evaluation of either time-of-flight mass spectra or mobility spectra are discussed here.
F. J. Knorr et al. (Anal. Chem. 1985, 57, 402; U.S. Pat. No. 4,633,083) have discussed a technique that operates in a time-of-flight dispersion spectrometer with an axial ion beam that is modulated by two barrier grids. According to the diagrams, the modulation function used is a square-wave function, i.e., an alternating complete closing and complete opening of the grid (a “binary function”). The first barrier grid is positioned directly behind the ion source, the second directly in front of the ion detector. Synchronous frequency modulation of both grids generates an interference value for the ion beam at which some ion species can pass through, while others are kept back by the interference of their drift time with the phases of the grid voltages. If this modulation frequency is altered, an interference spectrum (“interferogram”) can be acquired, which can be transformed by a Fourier transformation from the frequency domain of the interferogram into the time domain and thus into a mobility spectrum. The method, which has been called “Fourier Transform Ion Mobility Spectrometry” by its authors, offers a theoretical ion utilization factor of 25 percent because the ion quantities are halved at each of the two grids. However, expectations for this method in terms of increasing the signal-to-noise ratio have been disappointing, and the method has not gained acceptance up to now.
In U.S. Pat. No. 4,707,602 by F. J. Knorr, the second grid is replaced by a modulation of the detector current, or even modulation of the flow of the measured data, in order to generate the interference signal. Here, a square-wave modulation is shown in the diagrams, while the description also refers to a sinusoidal modulation. The utilization factor for the ions from the ion source is again 25 percent.
In order to produce clean interferograms with the two methods mentioned above, the modulation frequency must practically not vary during the time the ions drift from the first gating grid to the second gating grid or to the detector. This necessitates a slow change of the modulation frequency.
U.S. Pat. Nos. 5,396,065 and 6,198,096 disclose techniques for time-of-flight mass spectrometers (i.e., not for ion mobility spectrometers) which operate with very short pulses of ions. These pulses are stochastically distributed, with respect to time, with as high a density as possible so that the mass spectra of the pulses strongly overlap. The pre-determined pattern of the pulses forms a pseudo-stochastic sequence. The detector signal with the strongly overlapping mass spectra is evaluated by a correlation with the pattern of electric pulses at the gating grid, resulting in a well-resolved mass spectrum. In contrast to the patents described above, this method uses only one modulator for the ion current, a gating mechanism which only allows the passage of ions for a short time in the order of a few nanoseconds, and uses a correlation procedure to decode the ion current. It should be noted that a time-of-flight mass spectrometer does not have any diffusion broadening of the ion packages and that the detector signal is a simple superposition of the mass spectra of the individual ion pulses.
U.S. Pat. No. 5,719,392 discloses the ion current of an ion mobility spectrometer is modulated by the gating grid with a square-wave temporal Hadamard pattern, where both the pulse width of the admitted ion packages as well as their separations are statistically distributed. The evaluation to obtain the mobility spectrum can be done by either using a cross-correlation of the detector current and the impressed pattern, or by using Fourier or Hadamard transformations. Using the Fourier transformation makes it possible to obtain an improved mobility resolution by a partial deconvolution with the apparatus function. It has become apparent, however, that this evaluation procedure using the Fourier transformation does not operate stably for a noisy detector signal.
A cross-correlation in conjunction with a binary switched Hadamard pattern in a time-of-flight mass spectrometer is also used in U.S. Pat. No. 6,782,342.
U.S. Pat. No. 6,580,068 discloses an embodiment of the above-cited U.S. Pat. No. 4,707,602 expanded to all time-dispersive spectrometers. The modulation of the detector current, at least, has the function of a fast switch, and is thus binary, having the two states: admit and block. According to the authors, the modulation of the ion current of the ion source shall have the same shape, where possible, i.e., shall also be binary modulated. This patent cites a “chirp”, i.e., a temporal change of the modulation frequency from low to high frequencies, but uses only transformations of the interferogram from the frequency domain into the time domain and does not use cross-correlations.
There is a need for an improved technique of measuring the mobility spectrum of ions.